1. Field of the Invention
The present invention relates to a method of forming surface irregularities and a method of manufacturing a gallium nitride-based light emitting diode (hereinafter, referred to as a GaN-based LED), which can enhance light extraction efficiency of a vertical GaN-based LED using a GaN substrate.
2. Description of the Related Art
Generally, a GaN-based semiconductor LED is grown on a sapphire substrate, but the sapphire substrate is a rigid nonconductor and has poor thermal conductivity. Therefore, there is a limitation in reducing the manufacturing costs by decreasing the size of a GaN-based semiconductor LED, or improving the optical power and chip characteristic.
To solve this problem, there has been proposed a GaN-based LED using a GaN substrate with excellent conductivity and transmittance.
Such a GaN-based LED has surface irregularities formed on a light-emission surface, from which light is extracted, in order to enhance efficiency of light to be emitted from the GaN-based LED, that is, light extraction efficiency.
Now, a vertical GaN-based LED among conventional GaN-based LEDs using a GaN substrate will be described in detail with reference to FIG. 1. FIG. 1 is a perspective view of a conventional vertical GaN-based LED.
As shown in FIG. 1, the vertical GaN-based LED has a light-emission structure formed by sequentially forming an active layer 120 and a p-type nitride semiconductor layer 130 under a n-GaN substrate 110. Under the p-type nitride semiconductor layer 130, a positive electrode (p-electrode) 140 is formed. The n-GaN substrate 110 is formed to have a large thickness such that the active layer 120 and the p-type nitride semiconductor layer 130 are epitaxially grown and are supported by the n-GaN substrate 110.
The upper surface of the n-GaN substrate 110, that is, a light-emission surface of the light-emission structure has surface irregularities 150 for enhancing light extraction efficiency. On the surface irregularities 150, a negative electrode (n-electrode) 160 is formed.
The conventional vertical GaN-based LED employs the GaN substrate 110 having excellent conductivity and transmittance, instead of a sapphire substrate which is a rigid nonconductor and has poor thermal conductivity. Therefore, when the nitride semiconductor layers such as the active layer 120 and the p-type nitride semiconductor layer 130 are formed on the GaN substrate 110, lattice defects caused by growth can be minimized by the same lattice shape.
The GaN-based LED having the surface irregularities 150 operates as follows.
Holes injected through the p-electrode 140 spread laterally from the p-electrode 140. The holes are injected into the active layer 120 from the p-type nitride semiconductor layer 130. Electrons injected through the n-electrode 160 are injected into the active layer 120 from the n-GaN substrate 110. Then, the holes and the electrons are recombined in the active layer 120 so as to emit light. The light is emitted to the outside of the GaN-based LED through the surface irregularities 150 of the n-GaN substrate 110.
In order to minimize defects on the n-GaN substrate 110 serving as a light-emission surface, a wet-etching process is used to the surface irregularities 150 of the GaN-based LED, without using a dry-etching process where an occurrence rate of defects is high.
However, when the surface irregularities 150 are formed by using the wet-etching process, it is difficult to control the size and density of the surface irregularities 150, as shown in FIGS. 2 and 3.
FIGS. 2 and 3 are photographs for explaining the problems of the vertical GaN-based LED shown in FIG. 1. FIG. 2 shows a state where the density of the surface irregularities 150 is uneven. FIG. 3 shows a state where the sizes of the surface irregularities 150 are uneven.
As described above, when the sizes and density of the surface irregularities 150 are uneven, an improvement effect of light extraction efficiency is not enough.
Therefore, in this technical field, a new method is being required, which can minimize an occurrence rate of defects and maximize an improvement effect of light extraction efficiency.